A tire tread base rubber layer, which underlies the outer tread cap rubber layer is typically comprised of a unitary rubber composition. The rubber composition for the tread base rubber layer is often optimized to assist in promoting various overall tread properties such as, for example, reduced tire rolling resistance, tire handling such as cornering, reduced internally generated heat built up and to provide a physical transition between the outer tread cap rubber layer and the rest of the tire, particularly the supporting tire carcass. In practice, the tread base rubber layer may lie on a rubber reinforced cord belt beneath the tread or cord reinforced overlay or on a fabric reinforced rubber ply of the tire carcass.
Using the standard approach of a unitary base compound one could improve handling of a tire by using a stiffer base compound, which would also provide inferior rolling resistance. In contrast, one could improve rolling resistance by using a softer base which is less hysteretic and also this approach would provide worse handling performance. The desire to achieve improved handling and rolling resistance in a tire requires an alternative approach in tread base construction.
For this invention a departure from such past practice is contemplated in a sense of providing such tread base rubber layer with different zones of compound properties that would allow the opportunity to improve both rolling resistance and handling performance. This is accomplished by producing different zones of rubber composition properties in the tread base layer. The lateral tread base rubber zones would consist of higher stiffness rubber compositions than the central zone of the tread base layer. Handling performance for the tread and associated tire would be enhanced, since the lateral portion of the tread outer tread cap rubber layer immediately above and overlaying a lateral zone of the zoned tread base rubber layer would be supported by the lateral tread base zone which is stiffer than the central zone of the tread base rubber layer.
However at the same time, the tread outer running surface, whether in a cornering maneuver or simply rolling in a straight line, is required to go through a deformation process as the tread running surface comes in contact with the road surface and deforms the tread running surface to present a foot print shape. The softer center portion of the tread base can beneficially promote easier, lower energy demanding, and more consistent foot print shape deformation of the tire tread by providing a softer hinge point or center point of the tread for the tread bending taking place to assume the tire running surface's foot print shape. This associated reduced energy loss of assuming the foot print shape for the rolling tread running surface may also beneficially promote reduced rolling resistance for the tire tread and ultimately the tire. This approach is believed to be quite novel in a sense that handling of the tire could be substantially maintained without significant increase in its rolling resistance and with an opportunity of lowering rolling resistance where, in some cases, both rolling resistance and handling could be both beneficially improved at the same time. It is envisioned that such results relating to handling and rolling resistance could not be achieved while using a unitary, non-zoned, outer tread cap rubber composition, or compound. The choice of tread cap rubber compounds is not a part of this invention, but one could envision the use of a wide range of tread cap compounds with different properties and in some cases the use of zoned tread cap rubber layer.
Each of the tread base rubber zones are contemplated as being relatively wide, and therefore individually comprise a significant portion of the tread base layer rather than a very thin rubber segment of a few millimeters in width. For the proposes of this invention, the central tread base rubber zone may constitute, a considerable width of the tread base layer such as, for example, from about 55 to about 80 percent of the width of the tread base rubber layer, or alternatively, a more narrow width in a range of, for example, of from about 10 to about 30 percent of the axial width of the tread base rubber layer and two adjoining individual lateral tread base rubber zones may collectively and correspondingly constitute the remainder of the axial width of the tread base rubber layer.
In the description of this invention, the terms “rubber” and “elastomer” where herein, are used interchangeably, unless otherwise provided. The terms “rubber composition”, “compounded rubber” and “rubber compound”, if used herein, are used interchangeably to refer to “rubber which has been blended or mixed with various ingredients and materials” and such terms are well known to those having skill in the rubber mixing or rubber compounding art.
In the description of this invention, the term “phr” refers to parts of a respective material per 100 parts by weight of rubber, or elastomer. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise provided. The terms “cure” and “vulcanize” may be used interchangeably unless otherwise provided.
In the description of this invention, the glass transition temperature (Tg) may be determined according to differential scanning calorimeter (DSC) analysis at a heating rate of 10° C. per minute.